Based on their ability to provide control over wavefront, polarization and spectrum of light fields while having just nanoscale thickness, optical metasurfaces are promising candidates for flat optical components. Typically, metasurfaces consist of designed metallic or dielectric scatterers, which are arranged in a planar fashion on a subwavelength scale. Notably, most metasurfaces realized so far were based on periodic arrangements of the individual building blocks. Disorder, for example in the positioning, shape, or orientation of its building blocks, was usually associated with a deterioration of the optical properties due to an increase of incoherent scattering. Consequently, disorder related research in metasurfaces mainly concentrated on the development of designs which are robust against deviations from a perfect ordered geometry [1,2]. However, more recently, researchers started recognizing the introduction of controlled disorder as a handle to engineer metasurfaces exhibiting specific optical properties not accessible with periodic arrangements. For example, the introduction of disorder can decrease unwanted anisotropy in the optical response [3] and it can enhance the channel capacity of wavefront shaping metasurfaces [4].
Here we investigate two different types of disordered metasurfaces. In a first study, we consider a chiral plasmonic metasurface consisting of twisted gold-nanorod dimers. Chiral metasurfaces and metamaterials were intensively studied in the past. Most prominently, they can exhibit huge optical activity [5] and were suggested for applications as polarizing elements [6,7] or nanophotonic sensors. Using polarization spectroscopy and interferometric white-light spectroscopy, we demonstrate that the introduction of rotational disorder at the unit-cell level enables the realization of chiral plasmonic metasurfaces supporting pure circular dichroism, i.e., which is not accompanied by linear birefringence. Importantly, we show experimentally that the polarization eigenstates of these metasurfaces, which coincide with the fundamental right- and left-handed circular polarizations, do not depend on the wavelength in the spectral range of interest. Thereby, our metasurfaces closely mimic the behaviour of natural chiral media, while providing a much stronger circular dichroism.
In a second study, we concentrate on disordered silicon metasurfaces exhibiting electric and magnetic dipolar Mie-type resonances [8]. Silicon metasurfaces exhibit very low absorption losses in the near-infrared spectral range, thereby opening the door to long-range in-plane interactions between the individual nanoresonators. We systematically investigate how the introduction of different types of positional disorder influences the complex transmittance spectra of these metasurfaces, showing that disorder provides an independent degree of freedom for engineering their spatial and spectral dispersion.
[1] C. Helgert et al., Phys. Rev. B 79, 233107 (2009).
[2] N. Papasimakis et al., Phys. Rev. B 80, 041102(R) (2009).
[3] S. S. Kruk et al., Phys. Rev. B 88, 201404(R) (2013).
[4] D. Veksler et al., ACS Photonics 2, 661 (2015).
[5] M. Decker et al., Opt. Lett. 35, 1593 (2010).
[6] J. K. Gansel et al., Science 325, 1513 (2009).
[7] Y. Zhao et al., Nat. Commun. 3, 870 (2012).
[8] M. Decker et al., Adv. Opt. Mater. 3, 813 (2015).

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